In primary, normal cells, oncogenic ras activates the Raf-MEK-ERK MARK pathway, which in turn triggers a tumor-suppression defense response known as premature senescence. As a result, additional genetic alterations are required to bypass the senescence response in order for ras to induce transformation. While the downstream effectors mediating the oncogenic activity of ras have been extensively investigated, relatively little is known about the cellular pathway that is essential for ras to induce senescence and the mechanisms by which ras-induced senescence is bypassed in human tumors. Our preliminary studies indicate that the p38 MARK pathway acts downstream of MEK to mediate ras-induced senescence. Thus, the p38 pathway may provide a tumor-suppressing function that limits the oncogenic potential of ras. In addition, p16INK4a a key mediator of senescence, is induced by activated p38 partly through mRNA stabilization. This has revealed a novel mode of p16 regulation in senescence. Moreover, analysis of the adenoviral oncoprotein E1A revealed that E1 A-mediated senescence bypass relied on its ability to inactivate p300/CBP and Rb proteins, and might involve inhibition of p38. The goal of this application is to delineate the signaling pathway that confers senescence and the tumor-suppressing function of p38, and to investigate the potential mechanisms by which ras-induced senescence is bypassed during transformation. First, the potential involvement of p38 downstream kinases PRAK and MK2, p90RSK and p53 will be examined, in order to determine the signaling components that initiate or mediate the tumor suppression function of p38. Second, the impact of p300/CBP inactivation on senescence bypass and ras-induced activation of the p38 pathway will be investigated. These studies will shed lights on the genetic alterations required to overcome ras-induced senescence in human tumors. Finally, the mechanism of p16INK4a mRNA stabilization during senescence will be investigated. Since p16 plays a central role in ras-induced senescence, these studies will identify important proteins that act downstream of the p38 signaling pathway to mediate premature senescence. Overall, since increasing amounts of evidence have suggested a tumor-suppressing function of the p38 pathway, studies proposed here may lead to the discovery of novel tumor suppressor genes and new targets for cancer prevention and therapy.